Nitrous oxide emission and fertiliser nitrogen efficiency in a tropical sugarcane cropping system applied with different formulations of urea

Nitrous oxide ([N.sub.2]O) emissions from sugarcane cropped soils are usually high compared with those from other arable lands. Nitrogen-efficient management strategies are needed to mitigate [N.sub.2]O emissions from sugarcane farming whilst maintaining productivity and profitability. A year-long field experiment was conducted in wet tropical Australia to assess the efficacy of polymer-coated urea (PCU) and nitrification inhibitor (3,4-dimethylpyrazole phosphate)-coated urea (NICU). Emissions of [N.sub.2]O were measured using manual and automatic gas sampling chambers in combination. The nitrogen (N) release from PCU continued for >5-6 months, and lower soil N[O.sub.3.sup.-] contents were recorded for ≥ 3 months in the NICU treatments compared with the conventional urea treatments. The annual cumulative [N.sub.2]O emissions were high, amounting to 11.4-18.2 kg [N.sub.2]O-N [ha.sup.-1]. In contrast to findings in most other cropping systems, there were no significant differences in annual [N.sub.2]O emissions between treatments with different urea formulations and application rates (0, 100 and 140 kg N [ha.sup.-1]). Daily variation in [N.sub.2]O emissions at this site was driven predominantly by rainfall. Urea formulations did not significantly affect sugarcane or sugar yield at the same N application rate. Decreasing fertiliser application rate from the recommended 140 kg N [ha.sup.-1] to 100 kg N [ha.sup.-1] led to a decrease in sugar yield by 1.3t [ha.sup.-1] and 2.21 [ha.sup.-1] for the conventional urea and PCU treatments, respectively, but no yield loss occurred for the NICU treatment. Crop N uptake also declined at the reduced N application rate with conventional urea, but not with the PCU and NICU. These results demonstrated that substituting NICU for conventional urea may substantially decrease fertiliser N application from the normal recommended rates whilst causing no yield loss or N deficiency to the crop. Further studies are required to investigate the optimal integrated fertiliser management strategies for sugarcane production, particularly choice of products and application time and rates, in relation to site and seasonal conditions. Additional keywords: controlled-release fertiliser, DMPP, greenhouse gas, [N.sub.2]O, nitrification inhibitor, urea.
Introduction The efficiency of fertiliser nitrogen (N) use by sugarcane (Saccharum officinarum L.) crops is generally low, with ~40-60% of the applied N lost from the plant-soil system (Vallis el [...]